And although there was chatter earlier this year that Shelby might face a primary challenge, the deadline to file in Alabama's Republican primary is just more than a week away.

Schumer and Shelby both hold influential roles in the Senate, and at the end of the third quarter, they had accumulated the two biggest campaign war chests.

But the amount of money they've bankrolled raises the question: What can they do with it when they eventually leave the Senate? Retiring senators often gradually draw down their campaign accounts; they fundraise less and start to dole out what they have to other candidates or to the party committees. Already, the senators who have announced they are leaving at the end of this Congress have less cash on hand than many of their colleagues.

When Sen. Barbara Boxer ran against Republican Carly Fiorina in 2010, she had $6.35 million in the bank at this time in the cycle. Nearing the end of her last term, she's reduced her account to an $87,000 balance.

Outgoing Senate Minority Leader Harry Reid finished the first quarter of the year, which ended just days after he announced his retirement, with $1.5 million cash on hand. He now has $540,000 in the bank.

With what money they have left, retiring Senators (and House members, who often have less cash on hand) have many options.

For six months after leaving office, senators can use their campaign cash to wind down their offices, which includes moving personal and office furniture back to their home states.

Most retiring senators transfer money to another authorized committee (if they're running for another federal office), contribute it to other candidates or start their own political action committees.

"It depends on how connected they want to remain with the party and with politics," said Larry Noble, former general counsel to the Federal Election Commission, now of the Campaign Legal Center.

Personal use of campaign cash is, of course, forbidden. (Recall former Sen. Larry Craig's attempt to pay legal expenses with campaign cash.) But tracking where the money ends up isn't always straightforward.

Candidates running for two different federal offices are required to maintain separate campaign accounts. A federal office-holder, Noble explained, cannot transfer money between them if he or she is "actively seeking" the offices at the same time.

According to the FEC , a federal office-holder is considered to be "actively seeking" office as long as he or she hasn't become ineligible for election, withdrawn from the race or filed a termination report.

Retiring senators often give money away to other candidates, but any donation is subject to regular FEC contribution limits. A retiring federal office-holder can only make an unlimited political contribution if they're handing over campaign cash to the state, local or national party.

As a five-term senator, Shelby has come under fire for dipping into his campaign cash to treat himself and his donors , while not giving back enough to his party . In the third quarter, Shelby's campaign contributed to the Alabama Republican Party Federal Account and various county Republican committees in Alabama. Donating money to charity is another way retiring senators enshrine their legacy. The only limitation there is that they cannot benefit personally from their charitable contribution; in other words, the charity cannot hire the former senator and pay them a salary from the contribution.

And sometimes, retiring office-holders just sit on their campaign cash — for as long as they want. That's legal, too.

At the close of the third quarter, Democratic former Indiana Sen. Evan Bayh, who retired in 2011, had more than $9 million in the bank. Keeping that money on hand fueled chatter earlier this year that he might try to run for his state’s open senate seat, but he told CQ Roll Call earlier this year that he’s not interested. The hefty sum has also fueled vice presidential speculation. When Bayh retired, he said he intended to “help like-minded Democrats,” but so far, he’s not stepping into Indiana’s race, where Democratic former Rep. Baron Hill is having trouble raising money compared to his Republican opponents. The only two candidates to receive Bayh’s help this past quarter were Virginia Sen. Tim Kaine, who’s not up until 2018, and Indiana businesswoman Shelli Yoder, who’s running a long-shot Democratic bid for the state’s solidly red 9th District. There's nothing stopping retiring office-holders from returning their campaign cash to their contributions. Unsurprisingly, that rarely happens, Noble said.